In travelling wire EDM apparatus wherein a workpiece is cut by machining electrical discharges by means of an electrode wire, the cutting speed is limited by the thermal stresses exerted on the wire in the course of machining. The stresses are due on one hand to the heat produced by each electrical discharge and on the other hand to the ohmic losses of the machining current flowing along the wire. An important portion of the heat produced during machining is evacuated by the machining fluid injected between the wire and the workpiece in the machining zone; however there is a limit to the amount of heat that the machining fluid can handle. The only source of heat on which it is possible to act in order to increase the machining speed is the heat due to ohmic losses, or Joule effect.
It is conventional to supply the machining current to the electrode wire via two contacts, one located on one side and the other on the other side of the machining zone. The power dissipated as heat depends from the position where an electrical discharge occurs along the machining zone, the power being minimum when the electrical discharge occurs at one end of the machining zone and maximum when the electrical discharge appears at the middle of the machine zone. However, when an electrical discharge is produced proximate one end of the machining zone, the portion of machining current flowing from the other contact through the length of electrode wire in the machining zone simply produces a useless heating of the wire.
In Japanese Patent publication No. A59-47 123 there is disclosed an arrangement for supplying the machining current at one or the other of the ends of the machining zone at a frequency lower than that of the machining pulses. This enables a relative cooling of each of the halves of the machining zone to take place during the half period during which current is brought through the other end of the machining zone, but the total thermal losses are reduced only by a fraction.